The cellular aging process has been attributed to the accumulation of errors in cell control mechanisms with time. One possible and attractive hypothesis is that errors occurring in particular genes which control cell proliferation may cause cellular aging. To elucidate what type and amount of errors (mutations) are actually taking place in DNA molecules, we have introduced a shuttle vector pMCiI into early passage human diploid cells. By returning the vector to E. coli, we can estimate how many and what kind of mutations the vector has acquired in cultured cells.